Real-time X-ray diffraction to investigate elastic-plastic deformation in shocked magnesium doped lithium fluoride

X-ray diffraction measurements were carried out to examine lattice compression during time-dependent, elastic-plastic deformation in Mg-doped (∼100 ppm) LiF single crystals shocked along the [100] direction. Single and multiple diffraction measurements were used to determine lattice compression in the peak stress state (1.5 to 3 GPa) achieved in the LiF following reflection of the shock wave from an x-ray window. Time-resolved diffraction measurements (2 ns resolution) were used to obtain diffraction data corresponding to the elastic-plastic transition in crystals shocked to 3 GPa, as the shock wave traversed the x-ray probe region. Simultaneous wave profile measurements were obtained to ensure good correlation between the time-resolved diffraction and continuum data.;As expected, for the peak stresses achieved in these experiments, wave profiles for the doped crystals consisted of a large amplitude elastic wave followed by stress relaxation and a subsequent plastic wave. Single diffraction data obtained from the (200) lattice planes in combination with the macroscopic density compression demonstrate isotropic lattice compression in the peak state. Multiple diffraction data from the (200) and (202) planes provided direct determination of the lattice compression and verified the single diffraction results. The isotropic compression of the doped crystals is similar to previously reported results on ultrapure LiF and demonstrates that the large elastic wave amplitudes, due to the Mg impurities, do not affect the lattice compression in the peak state.;Time-resolved diffraction data from the (200) lattice planes showed an initial peak shift corresponding to the elastic wave front, followed by a smearing of the diffraction data during the elastic-plastic transition. Analysis of these data required the use of an analytic model and continuum calculations. The numerical simulations demonstrated uniaxial lattice compression at the elastic wavefront followed by a transition toward isotropic unit cell compression. Furthermore, it was concluded that the calculated stress deviators must diminish more rapidly than the current continuum model allows to match the diffraction data.